1. Field of the Invention
The present invention relates to densified ceramic bodies for electronic components and, in particular, to a densified ceramic body which is relatively free of surface defects.
2. Description of Related Art
Aluminum nitride (AlN) is of great interest as a material for electronic packages because of its high thermal conductivity and close match in thermal expansion to silicon. AlN can be densified at relatively low temperatures (1500-1700.degree. C.) by additions of various low-melting-temperature compounds. Because of its high equilibrium vapor pressure, densification of the AiN ceramic can only be accomplished by hot pressing or liquid phase sintering.
Electronic packages using AlN as a dielectric are typically processed from ceramic tape or greensheets. Greensheets consist of aluminum nitride powder, sintering aids, and an organic binder. The electrical conductor in the package is formulated into a paste made of refractory metal powder, ceramic additives, and organic binder. These two components are used in the process illustrated in FIGS. 1A-1D. The paste 12, 14 is screened essentially in a blanket pattern on the top sheet 16 of the greensheet stack 10, as illustrated in FIG. 1A, where up to 80% of the surface of the greensheet may be covered by the paste. Often, two different pastes are deposited, one on top of the other. One paste may be added to provide adhesion to the ceramic and another to offer a surface which has sufficient exposed metal to allow plating. Greensheets are provided with through holes called vias which are also filled with conductive paste. The sheets 16 are then laminated by applying sufficient heat and pressure and the result is illustrated in FIG. 1B. The laminated stack 18 is then heated to remove the organic binder from the paste 12,14 and from the greensheet 16. Further heating sinters the powders of both the paste and the greensheet. The resulting stack is illustrated in FIG. 1C. In the sinter cycle, the sintering aids form a liquid which further aids densification of the ceramic body 20. If any component of the liquid sintering aids has a high vapor pressure, the liquid phase will tend to wick to the surface(s) of the part. The surface of the part may then tend to have an accumulation of sintering aid byproducts. The byproducts may be in the form of particles 22 which protrude above the surface as illustrated in FIG. 1D. The protruding particles 22 are especially undesirable on the surface of an electronic chip carrier because the particles present a debris hazard in clean room operations and may also damage a chip during the chip attach process. The chip damage problem is especially exaggerated when the metal covers a high area percent of the ceramic.
Processing of electronic packages is greatly aided by use of a multilayer ceramic (MLC) technology, in which greensheets are cast using AlN and metallization is applied in the form of thixotropic ink or paste. The ink typically contains a solvent, a metal powder, a binder, and ceramic additives. The particle size of the metal powder and the volume fraction of the ceramic additives are chosen in order to assure a shrinkage match between the metallization and the ceramic. The ceramic additives are typically similar to that of the powder mixture in the greensheet. In cases where there is a surface metal feature to which IO devices (wire bonds, pins, tape automated bonding (TAB), etc.) will be attached, mechanical adhesion between the metal and the ceramic is required. Here, ceramic additives to the ink are also of great value, in that they may provide an interlocking feature between the ceramic and metal.
In some electronic packaging applications, however, there is a need for coverage of much of the surface of the ceramic with metallization. It is possible for 70% or more of the ceramic surface to be taken up by metal features. The specifications for the surfaces of these packages have very stringent limits on the height of bumps or bulges in the ceramic, in order to prevent damage to die which are later attached to the packages. There is also a strong desire to limit the potential for the ceramic to be a source of particulate contamination in a clean room environment.
In a package with high surface metal loading, however, a problem with the sintering aid may occur. The sintering aid may have a tendency to move toward the surface during the sinter cycle. Since a highly loaded system will have few areas which are not covered by metallization, the liquid phase may accumulate in these areas. Furthermore, the liquid phase is likely to decompose into phases which are of a high hardness. This decomposed phase may cause an electronic package to violate specifications for surface defect height or to cause concerns about clean room contamination. In some extreme cases, the surface second phase may cause bulges in the metallization.
Accordingly, it is highly desirable to develop aluminum nitride bodies with minimal surface defects.
Bearing in mind the problems and deficiencies of the prior art, it is therefore an object of the present invention to provide a ceramic surface for an electronic component whereby the surface is substantially free of surface defects.
It is another object of the present invention to provide an aluminum nitride body with minimal surface defects.
A further object of the invention is to provide a method of producing a ceramic body which is substantially free of surface defects.
It is yet another object of the present invention to provide a method of producing an aluminum nitride body with minimal surface defects.
Still other objects and advantages of the invention will in part be obvious and will in part be apparent from the specification.